Keith Devens .com

Entry 1489

If anyone does a lot of web-based programming, please help.

You know how on real e-commerce sites like Amazon you can go back and forth in your browser, even after you've submitted a form? How do they do that? I usually get "page expired" messages for my own code in certain circumstances. Do they just enable a high level of caching, or do they structure their site differently?

Also, what algorithm do you use to validate credit card numbers? I know there are complicated checks you can do, but all I'm sure about now is that they have to be numeric and at least 16 characters. I know each credit card company has their own prefix (Discover cards start with 6, MasterCards start with 5, Visa's start with 4, etc.), but is there any documentation from the companies explaining their numbering scheme?

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